Signals are widely used to represent and carry information. Signals are typically propagated in a medium, such as an electrical charge in a wire or radio waves in the air. Signals generally come in two forms, digital signals and analog signals. Digital signals are discrete and quantized, while analog signals are continuous. Both of these types of signals may be modified by using devices such as amplifiers, filters, and signal processors. These devices can change the amplitude of a signal or some frequencies of a signal. Devices such as analog-to-digital (A/D) converters and digital-to-analog (D/A) converters change a signal from being analog to digital and vice versa. Typically, to perform this conversion, the signal is measured at time instants, which is usually referred to as sampling, and then assigned a discrete value after a comparison is performed.